Process and apparatus for producing glass sheet

ABSTRACT

Provided is a process for producing a glass sheet including a forming step of down-drawing a molten glass into a sheet-like glass ribbon, in which the molten glass is fed to a forming trough arranged in a forming furnace and the molten glass is caused to flow down from the forming trough through a conveyance passage extending vertically; an annealing step of removing an internal strain in the glass ribbon in an annealing furnace; a cooling step of cooling the glass ribbon to around room temperature in a cooling chamber; and a cutting step of cutting the glass ribbon in a given size, in which the cooling chamber is provided with a gas exhausting passage, thereby exhausting air in the cooling chamber to an outside.

TECHNICAL FIELD

The present invention relates to a process and apparatus for producing aglass sheet, the process involving causing a molten glass to flow downfrom a forming trough, and vertically down-drawing a glass ribbon.

BACKGROUND ART

As a process for producing a glass sheet for various electronicinstruments, in particular, a flat panel display such as a liquidcrystal display, there is known a down-draw method, in which a glasssheet is produced by causing a molten glass to flow down from a formingtrough, and vertically down-drawing a glass ribbon.

The down-draw method includes two methods, i.e., an overflow down-drawmethod and a slot down-draw method. In particular, the overflowdown-draw method is widely known as a method, with which a glass sheethaving very small waviness and roughness on its surface and beingexcellent in surface quality can be obtained.

The overflow down-draw method is a method, which involves continuouslyfeeding a molten glass to a top portion of a forming trough having awedge-shaped cross-section, causing the molten glass to flow down fromthe top portion of the forming trough along both side surfaces of theforming trough, and allowing the molten glasses to fuse at a lower endportion of the forming trough to form a sheet-like glass ribbon, andcausing the glass ribbon to flow down through a conveyance passageextending vertically while holding the glass ribbon at both edgeportions with a plurality of pulling rollers, thereby down-drawing themolten glass into a glass ribbon. With this process, the glass ribbon isgradually solidified, and a glass sheet having a given width andthickness is obtained. Further, atmospheric temperature in theconveyance passage is strictly controlled, and the internal strain(thermal strain) in the glass sheet is sufficiently reduced thereby.Then, the glass sheet is cooled to around room temperature.

In a case of a glass sheet for a liquid crystal display, in particular,if even only a minute internal strain remains in the glass sheet,birefringence occurs, and as a result, a uniform image is not provided.Thus, there have conventionally been proposed various ideas foruniformly cooling a glass ribbon as much as possible at a giventemperature gradient.

For example, JP-A-5-124826 discloses a structure including supporting aroller on a single side for preventing occurrence of an internal strainin a glass sheet caused by the influence of the cooling of a rollershaft so as to prevent the deformation of the glass sheet, and alsodiscloses a preventive plate against convection for separating aconveyance passage horizontally for preventing the internal strain in aglass ribbon caused by thermal convection occurring in the conveyancepassage.

Further, JP-A-10-53427 discloses a process for producing a glass sheethaving a small internal strain, the process involving forming aplurality of chambers by separating an internal space of a formingfurnace or an annealing furnace horizontally, and allowing each chamberto have a room temperature-controlling function to carry out sufficientannealing.

Still further, JP-A-2001-31435 discloses a technology by whichtemperature distribution in the annealing furnace is also formed in awidth direction of a glass ribbon, to thereby prevent a minute internalstrain or deformation.

Patent Document 1: JP-A-5-124826 Patent Document 2: JP-A-10-53426 PatentDocument 3: JP-A-2001-31435 DISCLOSURE OF THE INVENTION Problem to beSolved by the Invention

In recent years, high definition and high quality are increasinglydemanded for a liquid crystal display, and it is demanded for a glasssheet used for the liquid crystal display to have a maximum value of aninternal strain of 1.0 MPa or less. Further, a glass sheet for a liquidcrystal display is rapidly moving toward a large-size plate. Forexample, even a glass ribbon having a width of 2000 mm or more as thewidth size of a portion serving as a final glass product (effectivewidth) is produced. However, as the size of a glass sheet produced isbecoming larger, an internal strain in the glass sheet also tends tobecome larger, and hence it is becoming difficult to control theinternal strain to 1.0 MPa or less.

One of the causes for the internal strain in a glass sheet is flow ofair climbing along the surface of a glass ribbon (hereinafter referredto as low-temperature airflow). That is, in a conveyance passage of aglass ribbon, low-temperature airflow always climbs along the surface ofthe glass ribbon, causing easy variation of the atmospheric temperaturein an annealing furnace. JP-A-5-139766 discloses formation of apreventive plate against convection in an annealing furnace, but becauselow-temperature airflow climbs near the surface of a glass ribbon, thepreventive plate against convection cannot cut off the low-temperatureairflow sufficiently. Further, when a preventive plate againstconvection is used to fully block low-temperature airflow, it isnecessary for the distance between a glass ribbon and the preventiveplate against convection to be very small, but in this case, the glassribbon contacts the preventive plate against convection, thereby causingpossible formation of flaws on the surface of the glass ribbon.

The present invention has been made in view of the above-mentionedcircumstances. A technological object of the present invention is toprovide a process for obtaining a glass sheet of high quality with goodproductivity by avoiding a problem with an internal strain that becomesserious as the move toward a large-size glass sheet progresses.

Means for Solving the Problem

The inventors of the present invention have made various studies tosolve the above-mentioned problem. As a result, the inventors have foundthat the climb of low-temperature airflow can be suppressed in theconveyance passage of a glass ribbon by providing a cooling chamber witha gas exhausting passage, thereby reducing the amount of thelow-temperature airflow flowing from the cooling chamber to theannealing furnace. Thus, the present invention has been proposed.

The invention according to claim 1 made for solving the above-mentionedproblem relates to a process for producing a glass sheet including: aforming step of down-drawing a molten glass into a sheet-like glassribbon, in which the molten glass is fed to a forming trough arranged ina forming furnace and the molten glass is caused to flow down from theforming trough through a conveyance passage extending vertically; anannealing step of removing an internal strain in the glass ribbon in anannealing furnace; a cooling step of cooling the glass ribbon to aroundroom temperature; and a cutting step of cutting the glass ribbon in agiven size, in which the cooling chamber is provided with a gasexhausting passage, thereby exhausting air in the cooling chamber to anoutside.

The invention according to claim 2 made for solving the above-mentionedproblem relates to the process for producing a glass sheet according toclaim 1 further including exhausting the air in the cooling chamber intoa chamber surrounding a forming furnace and/or an annealing furnacethrough the gas exhausting passage.

The invention according to claim 3 made for solving the above-mentionedproblem relates to the process for producing a glass sheet according toclaim 1 or 2, in which the forming step includes a step of forming aglass ribbon by an overflow down-draw method or a slot down-draw method.

The invention according to claim 4 made for solving the above-mentionedproblem relates to the process for producing a glass sheet according toany one of claims 1 to 3, in which a length of a short side of the glasssheet is 2000 mm or more.

The invention according to claim 5 made for solving the above-mentionedproblem relates to the process for producing a glass sheet according toany one of claims 1 to 4, in which a maximum value of the internalstrain of the glass sheet is 1.0 MPa or less.

The invention according to claim 6 made for solving the above-mentionedproblem relates to the process for producing a glass sheet according toany one of claims 1 to 5, in which the glass sheet contains, in terms ofmass %, 40 to 70% of SiO₂, 2 to 25% of Al₂O₃, 0 to 20% of B₂O₃, 0 to 10%of MgO, 0 to 15% of CaO, 0 to 10% of SrO, 0 to 15% of BaO, 0 to 10% ofZnO, 0 to 10% of ZrO₂, and 0 to 2% of an fining agent.

The invention according to claim 7 made for solving the above-mentionedproblem relates to an apparatus for producing a glass sheet including: aforming furnace for down-drawing a molten glass into a sheet-like glassribbon, in which the molten glass is fed to a forming trough and themolten glass is caused to flow down from the forming trough through aconveyance passage extending vertically; an annealing furnace forremoving an internal strain in the glass ribbon; a cooling chamber forcooling the glass ribbon to around room temperature; and a cuttingchamber for cutting the glass ribbon in a given size, in which thecooling chamber is provided with a gas exhausting passage.

The invention according to claim 8 made for solving the above-mentionedproblem relates to the apparatus for producing a glass sheet accordingto claim 7, in which the gas exhausting passage of the cooling chamberis communicated with a chamber surrounding a forming furnace and/or anannealing furnace.

EFFECTS OF THE INVENTION

In the process for producing a glass sheet, which include a forming stepof down-drawing a molten glass into a sheet-like glass ribbon, in whichthe molten glass is fed to a forming trough arranged in a formingfurnace and the molten glass is caused to flow down from the formingtrough through a conveyance passage extending vertically, an annealingstep of removing an internal strain in the glass ribbon in an annealingfurnace, a cooling step of cooling the glass ribbon to around roomtemperature, and a cutting step of cutting the glass ribbon in a givensize, the invention according to claim 1 provides the cooling chamberwith a gas exhausting passage to exhaust the air in the cooling chamberto the outside, to thereby allow the air in the cooling chamber to beexhausted while being dispersed into both of the conveyance passage ofthe glass ribbon and the gas exhausting passage, with the result thatthe climb of low-temperature airflow can be suppressed in the conveyancepassage. As a result, the variation of the atmospheric temperature inthe annealing furnace can be suppressed to a minimum extent, leading toa sufficient reduction in the internal strain in a glass sheet even ifthe size of the glass sheet becomes large. The cooling chamber ispreferably provided with the gas exhausting passage at its ceilingportion because the air-exhausting effect becomes larger.

The invention according to claim 2 allows the air in the cooling chamberto be exhausted to a chamber surrounding a forming furnace and/or anannealing furnace through the gas exhausting passage, to thereby elevatethe pressure in the chamber surrounding a forming furnace and/or anannealing furnace. As a result, an effect of suppressing the climb ofthe low-temperature airflow in the conveyance passage of the glassribbon becomes larger. That is, the low-temperature airflow that hasclimbed from the cooling chamber into the annealing furnace is heated inthe annealing furnace, and then part of the low-temperature airflowleaks into the outside atmosphere through the gaps of furnace walls ofthe forming furnace and/or an annealing furnace. However, the elevationof the pressure in the chamber surrounding a forming furnace and/or anannealing furnace suppresses the leakage of the inside air of theforming furnace and/or annealing furnace. As a result, the effect ofsuppressing the climb of the low-temperature airflow in the conveyancepassage of the glass ribbon becomes larger.

By using the invention according to claim 3, the forming step is a stepfor forming a glass ribbon by an overflow down-draw method or a slotdown-draw method and hence a thin plate glass can be efficiently formed.In order to obtain a plate glass particularly excellent in surfacequality, it is desirable to employ the overflow down-draw method ratherthan the slot down-draw method. It should be noted that the slotdown-draw method is a method involving supplying a molten glass to aforming trough having an aperture in a long-hole shape (slot shape),then pulling the molten glass out of the aperture of the forming troughto form a sheet-like glass ribbon, and vertically extending and formingthe glass ribbon into a glass sheet.

Further, in the present invention, when the glass ribbon is cut in agiven size, the glass ribbon flowing down vertically may be cut in thewidth direction of the glass ribbon (direction perpendicular to theflowing-down direction of the glass ribbon) from the cooling step, orthe glass ribbon may be bent from the vertical direction to thehorizontal direction and cut in the width direction while being moved inthe horizontal direction.

By using the invention according to claim 4, the glass sheet has alength of the short side of 2000 mm or more, and hence many glass sheetsfor a display panel can be cut out from one piece of the glass sheet(original sheet), enabling the improvement in the production efficiency.The larger the size of the glass sheet, the larger the internal strainof the glass sheet tends to be. One of the reasons for that isconsidered to be that when the size of the glass sheet becomes larger,the apparatus for producing a glass sheet has to be larger, resulting inthe easiness for low-temperature air to flow in the conveyance passageof the glass ribbon, with the result that the variation of theatmospheric temperature in the annealing furnace easily occurs. Thus,the present invention becomes useful when the invention is used forproducing particularly large glass sheets, specifically, a glass sheethaving a length of the short side of 2000 mm or more, preferably 2500 mmor more, or still more preferably 3000 mm or more.

By using the invention according to claim 5, the glass sheet has amaximum value of an internal strain of 1.0 MPa or less, and hence theimage of a liquid crystal display is prevented from becoming nonuniformdue to birefringence. By using the present invention, the variation ofthe atmospheric temperature in the annealing furnace can be suppressedto a minimum extent, leading to the suppression of the internal strainin the glass sheet even if the size of the glass sheet becomes large. Tobe specific, it becomes possible to control the maximum value of theinternal strain to 1.0 MPa or less, 0.8 MPa or less, or further 0.7 MPaor less.

By using the invention according to claim 6, the glass sheet contains,in terms of mass %, 40 to 70% of SiO₂, 2 to 25% of Al₂O₃, 0 to 20% ofB₂O₃, 0 to 10% of MgO, 0 to 15% of CaO, 0 to 10% of SrO, 0 to 15% ofBaO, 0 to 10% of ZnO, 0 to 10% of ZrO₂, and 0 to 2% of a fining agent,and hence the glass sheet satisfies characteristics such as chemicalresistance (good acid resistance, good alkali resistance, and goodbuffered-hydrofluoric-acid resistance), thermal resistance (strain pointof 630° C. or more), meltability (1600° C., temperature corresponding toa viscosity of 10^(2.5) poise), formability (liquidus temperature of1150° C. or less), and thermal expansion coefficient (25 to 45×10⁻⁷/° C.at a temperature of 30 to 380° C.). Thus, it is possible to obtain aglass sheet for a liquid crystal display in which the internal strainafter formation can be easily suppressed.

The reason why the above-mentioned glass composition is preferred is asfollows.

SiO₂ is a component for forming the network of glass and has effects ofreducing the thermal expansion coefficient of glass, making the internalstrain smaller, improving the acid resistance of glass, and elevatingthe strain point of glass to make the thermal contraction of a glasssheet smaller. However, when the content of SiO₂ becomes large, theviscosity of glass becomes higher, with the result that meltabilitytends to deteriorate and devitrified stones of cristobalite tends toprecipitate. Thus, the content of SiO₂ is 40 to 70%, preferably 50 to67%, or more preferably 57 to 64%.

Al₂O₃ is a component for reducing the thermal expansion coefficient ofglass and for making the internal strain of a glass sheet smaller.Further, Al₂O₃ has effects of elevating the strain point of glass andsuppressing the precipitation of devitrified stones of cristobalite aswell. However, when the content of Al₂O₃ becomes large, thebuffered-hydrofluoric-acid resistance of glass deteriorates and liquidustemperature rises, resulting in a difficulty in formation. Thus, thecontent of Al₂O₃ is 2 to 25%, preferably 10 to 20%, or more preferably14 to 17%.

B₂O₃ is a component, acting as a melting accelerate component, forreducing the viscosity of glass and for improving the meltability.Further, B₂O₃ is a component for reducing the thermal expansioncoefficient of glass and for making the internal strain of a glass sheetsmaller. However, when the content of B₂O₃ becomes large, the strainpoint of glass easily lowers and the acid resistance easilydeteriorates. Thus, the content of B₂O₃ is 0 to 20%, preferably 5 to15%, or more preferably 7.5 to 12%.

MgO is a component for improving the meltability of glass by reducingonly the viscosity of the glass without reducing the strain point.However, when the content of MgO becomes large, devitrified stoneseasily precipitate in glass, and buffered-hydrofluoric-acid resistancelowers, with the result that when a glass sheet is treated with abuffered hydrofluoric acid, the surface of the glass sheet is corroded,a reaction product attaches to the surface, and the surface easilybecomes clouded. Thus, the content of MgO is 0 to 10%, preferably 0 to5%, or more preferably 0 to 3.5%.

CaO is a component for improving the meltability of glass by reducingonly the viscosity of the glass without reducing the strain point.However, when the content of CaO becomes large,buffered-hydrofluoric-acid resistance easily deteriorates. Thus, thecontent of CaO is 0 to 15%, preferably 0 to 12%, or more preferably 3.5to 9%.

SrO is a component for enhancing the chemical resistance anddenitrification resistance of glass. However, when the content of SrObecomes large, the thermal expansion coefficient of glass easily becomeslarge and the internal strain of a glass sheet tends to become large.Thus, the content of SrO is 0 to 10%, preferably 0 to 8%, or morepreferably more than 0.5 to 8%.

BaO is a component for enhancing the chemical resistance anddenitrification resistance of glass similar to SrO. However, when thecontent of BaO becomes large, the density and thermal expansioncoefficient of glass tends to become large, and meltability of glasstends to remarkably deteriorate. Thus, the content of BaO is 0 to 15%,preferably 0 to 10%, or more preferably 0 to 8%.

ZnO is a component for improving the buffered-hydrofluoric-acidresistance and meltability of glass. When the content of ZnO becomeslarge, the denitrification resistance and strain point of glass easilylowers. Thus, the content of ZnO is 0 to 10%, preferably 0 to 5%, ormore preferably 0 to 1%.

ZrO₂ is a component for raising the strain point of glass. When thecontent of ZrO₂ becomes large, the density of glass remarkablyincreases, and devitrified stones derived from ZrO₂ easily precipitate.Thus, the content of ZrO₂ is 0 to 10%, preferably 0 to 7%, or morepreferably 0 to 5%.

As₂O₃, Sb₂O₃, SnO₂, SO₃, F, Cl, or the like can be used as a finingagent up to 2%. It should be noted that the use of As₂O₃ and Sb₂O₃should be avoided because they are environmental load substances, andwhen the use of As₂O₃ and Sb₂O₃ is avoided, SnO₂ is contained preferablyat 0.01 to 2%.

In addition, in the present invention, it is possible that, other thanthe above-mentioned components, for example, up to 3% each of Y₂O₃,La₂O₃, Nb₂O₃, and P₂O₅ can be contained for reducing the liquidustemperature of glass to improve the formability. It should be noted thatalkali metal oxides (R₂O) such as Na₂O, K₂O, and Li₂O should not becontained because when those components are contained, thecharacteristics of various films and TFT devices formed on the glasssheet for a liquid crystal display may be degraded. To be specific, thecontent in terms of R₂₀ must be regulated at 0.1% or less.

By using the invention according to claim 7, in which an apparatus forproducing a glass sheet includes a forming furnace for down-drawing amolten glass into a sheet-like glass ribbon, in which the molten glassis fed to a forming trough and the molten glass is caused to flow downfrom the forming trough through a conveyance passage extendingvertically, an annealing furnace for removing an internal strain in theglass ribbon; a cooling chamber for cooling the glass ribbon to aroundroom temperature; and a cutting chamber for cutting the glass ribbon ina given size, the cooling chamber being provided with a gas exhaustingpassage to exhaust the air in the cooling chamber to the outside, tothereby allow the air in the cooling chamber to be exhausted while beingdispersed into both of the conveyance passage of the glass ribbon andthe gas exhausting passage, with the result that the climb oflow-temperature airflow can be suppressed in the conveyance passage. Asa result, the variation of the atmospheric temperature in the annealingfurnace can be suppressed to a minimum extent, leading to a sufficientreduction in the internal strain in a glass sheet even if the size ofthe glass sheet becomes large.

In the invention according to claim 8, because the gas exhaustingpassage of the cooling chamber is communicated with the chambersurrounding a forming furnace and/or an annealing furnace, the air inthe cooling chamber is allowed to flow into the chamber surrounding aforming furnace and/or an annealing furnace, causing the elevation ofthe pressure in the chamber, to thereby provide a rare possibility forthe inside air in the forming furnace and/or annealing furnace to leakinto the outside through the gaps of furnace walls of the furnaces. As aresult, an effect of suppressing the climb of the low-temperatureairflow in the conveyance passage of the glass ribbon becomes larger.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention is described indetail in reference to a figure attached.

FIG. 1 is a schematic front view illustrating an apparatus for producinga glass sheet of the present invention. The production apparatus is forproducing a glass sheet (glass substrate) for a liquid crystal displayby an overflow down-draw method. In the order from the top, theproduction apparatus is provided with a forming furnace 11 for forming aglass ribbon B by overflowing a molten glass A supplied to a formingtrough 10 having a wedge-shaped cross-section from the top portion ofthe forming trough 10 and allowing the molten glass A to fuse at thelower end portion of the forming trough 10, an annealing furnace 12 forremoving the internal strain in the glass ribbon B while annealing theglass ribbon B, a cooling chamber 13 for sufficiently cooling the glassribbon B annealed, and a cutting chamber 14 for cutting the glass ribbonB cooled in a given size. In addition, the cooling chamber 13 isprovided with a gas exhausting passage 15 at its ceiling portion. Theforming furnace 11 and the annealing furnace 12 are surrounded by aforming chamber 16. The cooling chamber 13 and the forming chamber 16are communicated with each other via the gas exhausting passage 15. Thecooling chamber 13, the cutting chamber 14, and the forming chamber 16,which are neighboring in the vertical direction, are surrounded by aperipheral wall portion 17 having airtightness. The forming furnace 11,the annealing furnace 12, the cooling chamber 13, and the cuttingchamber 14 are communicated with each other via a conveyance passage 18through which the glass ribbon B flows down. Further, the cuttingchamber 14 is provided with another conveyance passage for conveying aglass sheet C to a subsequent step (for example, edge-polishing step)which is not shown.

Next, a production process for a glass sheet using the above-mentionedapparatus for producing a glass sheet is described.

In the production apparatus, the molten glass A is first supplied to thetop portion of the forming trough 10 provided in the forming furnace 11,the molten glass A is caused to overflow from the top portion of theforming trough 10, and the molten glass A is fused at the lower endportion of the forming trough 10, to thereby form into a sheet-likeglass ribbon B. In the vicinity of the forming trough 10, a pair ofcooling rollers (edge rollers) 19 are provided, and the cooling rollers19 hold both edges of the glass ribbon B, thereby suppressing itscontraction in the width direction to a minimum extent.

Next, annealing the formed glass ribbon B in the annealing furnace 12removes its internal strain. The annealing furnace 12 is provided with aplurality of pairs of pulling rollers (annealing roller) 19 in thevertical direction, and the glass ribbon B is pulled downward while thepulling rollers 20 are pulling the glass ribbon B in the width directionto prevent the glass ribbon B from contracting in the width directionbecause of surface tension or the like. In addition, the annealingfurnace 12 is set so as to have a given temperature gradient controlledby a heater (not shown). Thus, the temperature of the glass ribbon B isgradually lowered as the glass ribbon B flows down through the annealingfurnace 12, thereby removing the internal strain.

In addition, the cooling chamber 13 in the downstream of the annealingfurnace 12 is provided with a plurality of pairs of supporting rollers21, which pull downward the glass ribbon B solidified in a given widthand given thickness. The glass ribbon B is cooled to around roomtemperature in the cooling chamber 13. In addition, the air in thecooling chamber 13 flows into both the annealing furnace 12 and the gasexhausting passage 15, and the air that has flown into the gasexhausting passage 15 flows into the forming chamber 16. As a result,the amount of the air flowing into the annealing furnace 12 is reduced,suppressing the climb of the low-temperature airflow in the conveyancepassage 18 of the glass ribbon.

The glass ribbon cooled to around room temperature in the coolingchamber 14 is cut into glass sheets C having a given size in the cuttingchamber 14, and the glass sheets are conveyed to a subsequent step.

The above-mentioned apparatus for producing a glass sheet was used toform a glass sheet for a liquid crystal display containing, in terms ofmass %, 60% of SiO₂, 15% of Al₂O₃, 10% of B₂O₃, 6% of CaO, 6% of SrO, 2%of BaO, and 1% of an fining agent (OA-10, manufactured by NipponElectric Glass Co., Ltd.).

The dimension of the glass sheet obtained was 2360×2030×0.7 mm. Themaximum strain of the glass sheet was measured and was 0.8 MPa.

Further, FIG. 2 is a schematic front view illustrating an apparatus forproducing a glass sheet of a comparative example. The structure of theapparatus is the same as that of the apparatus in FIG. 1 except that acooling chamber 13 is not provided with a gas exhausting passage. Theapparatus in FIG. 2 was used to produce a glass sheet in the sameconditions as those in the above-mentioned embodiment. The maximumstrain of the glass sheet was measured and was 1.1 MPa.

The foregoing shows that the glass sheet obtained in the embodiment hasa smaller maximum strain than the glass sheet obtained in thecomparative example, and hence the present invention has a greatereffect of reducing the internal strain of a glass sheet by providing agas exhausting passage leading from the cooling chamber to a chambersurrounding the annealing furnace.

Here, the maximum strain of a glass sheet was determined by measuringstrain stress from the birefringence amount of the glass sheet throughan optical heterodyne interferometry with a strain indicatormanufactured by Uniopt Co., Ltd. The reason why the maximum strain of aglass sheet was determined is that if even only one strong strain ispresent in the glass sheet, the glass sheet does not meet the productspecification for a glass sheet for a liquid crystal display.

It should be noted that the present invention is not limited to theabove-mentioned embodiment, and may be carried out in any other variousembodiments as long as the embodiments do not deviate from the gist ofthe present invention.

For example, the above-mentioned embodiment described the case where thepresent invention was applied to the production of a glass sheet by anoverflow down-draw method. In addition to that, for example, the presentinvention can be likewise applied to the production of a glass sheet bya slot down-draw method.

Further, although the embodiment described the case where the formingfurnace and the annealing furnace were surrounded by one chamber(forming chamber), the forming furnace and the annealing furnace maybeeach surrounded by different chambers (for example, forming chamber andannealing chamber). In that case, the gas exhausting passage of thecooling chamber is provided so as to lead to the annealing chamber.

Further, although the embodiment described the case where the gasexhausting passage was provided near the annealing furnace, the gasexhausting passage may be provided apart from the annealing furnace. Inaddition, it is recommended that the shape and size of the gasexhausting passage be suitably set depending on the size of the coolingchamber and annealing furnace or the like.

INDUSTRIAL APPLICABILITY

The process and apparatus for producing a glass sheet of the presentinvention can be used for the production, mainly of a glass sheet for aliquid crystal display, of a glass sheet used for various flat paneldisplays, for example, a plasma display, an electroluminescence displaysuch as an OLED display, and a field emission display, and of a glasssheet used as a substrate on which various devices with an electronicdisplay function or various thin films are formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view illustrating an apparatus for producinga glass sheet of the present invention.

FIG. 2 is a schematic front view illustrating an apparatus for producinga glass sheet of a comparative example.

DESCRIPTION OF SYMBOLS

-   -   10 forming trough    -   11 forming furnace    -   11 a furnace wall of forming furnace    -   12 annealing furnace    -   12 a furnace wall of annealing furnace    -   13 cooling chamber    -   14 cutting chamber    -   15 gas exhausting passage    -   16 forming chamber    -   17 peripheral wall portion    -   18 conveyance passage    -   19 cooling roller (edge roller)    -   20 pulling roller (annealing roller)    -   21 supporting roller    -   22 air communication hole    -   A molten glass    -   B glass ribbon    -   C glass sheet

1. A process for producing a glass sheet, comprising: a forming step ofdown-drawing a molten glass into a sheet-like glass ribbon, in which themolten glass is fed to a forming trough arranged in a forming furnaceand the molten glass is caused to flow down from the forming troughthrough a conveyance passage extending vertically; an annealing step ofremoving an internal strain in the glass ribbon in an annealing furnace;a cooling step of cooling the glass ribbon to around room temperature ina cooling chamber; and a cutting step of cutting the glass ribbon in agiven size, wherein: the cooling chamber is provided with a gasexhausting passage, thereby exhausting air in the cooling chamber to anoutside.
 2. The process for producing a glass sheet according to claim1, further comprising exhausting the air in the cooling chamber into achamber surrounding a forming furnace and/or an annealing furnacethrough the gas exhausting passage.
 3. The process for producing a glasssheet according to claim 1, wherein the forming step comprises a step offorming a glass ribbon by an overflow down-draw method or a slotdown-draw method.
 4. The process for producing a glass sheet accordingto claim 1, wherein a length of a short side of the glass sheet is 2000mm or more.
 5. The process for producing a glass sheet according toclaim 1, wherein a maximum value of the internal strain of the glasssheet is 1.0 MPa or less.
 6. The process for producing a glass sheetaccording to claim 1, wherein the glass sheet contains, in terms of mass%, 40 to 70% of SiO₂, 2 to 25% of Al₂O₃, 0 to 20% of B₂O₃, 0 to 10% ofMgO, 0 to 15% of CaO, 0 to 10% of SrO, 0 to 15% of BaO, 0 to 10% of ZnO,0 to 10% of ZrO₂, and 0 to 2% of an fining agent.
 7. An apparatus forproducing a glass sheet, comprising: a forming furnace for down-drawinga molten glass into a sheet-like glass ribbon, in which the molten glassis fed to a forming trough and the molten glass is caused to flow downfrom the forming trough through a conveyance passage extendingvertically; an annealing furnace for removing an internal strain in theglass ribbon; a cooling chamber for cooling the glass ribbon to aroundroom temperature; and a cutting chamber for cutting the glass ribbon ina given size, wherein: the cooling chamber is provided with a gasexhausting passage.
 8. The apparatus for producing a glass sheetaccording to claim 7, wherein the gas exhausting passage of the coolingchamber is communicated with a chamber surrounding a forming furnaceand/or an annealing furnace.
 9. The process for producing a glass sheetaccording to claim 2, wherein the forming step comprises a step offorming a glass ribbon by an overflow down-draw method or a slotdown-draw method.
 10. The process for producing a glass sheet accordingto claim 2, wherein a length of a short side of the glass sheet is 2000mm or more.
 11. The process for producing a glass sheet according toclaim 2, wherein a maximum value of the internal strain of the glasssheet is 1.0 MPa or less.
 12. The process for producing a glass sheetaccording to claim 2, wherein the glass sheet contains, in terms of mass%, 40 to 70% of SiO₂, 2 to 25% of Al₂O₃, 0 to 20% of B₂O₃, 0 to 10% ofMgO, 0 to 15% of CaO, 0 to 10% of SrO, 0 to 15% of BaO, 0 to 10% of ZnO,0 to 10% of ZrO₂, and 0 to 2% of an fining agent.